1. Field of the Invention
The invention concerns a process for the production of water vapour-permeable sheet materials, preferably for such materials which are suitable for use in the footwear industry. The invention also concerns a meterial produced by the process and which has improved water-vapour permeability.
2. Description of the Prior Art
It is well-known that the use of synthetic leathers in substitution of natural leathers has grown significantly in order to satisfy increased demands and other trends in development.
It is also known that in the use of synthetic leathers, especially in the footwear industry, the unsatisfactory water-permeability of such materials causes a serious problem.
It is furthermore also known that a key question of the technology of producing synthetic leathers that have been developed in substitution of natural leathers is the problem of ensuring a satisfactory water-vapour permeability for the finished product. Having regard to the fact that in the case of synthetic leathers used in the footwear industry, the chief requirements are good vapour-permeability and water vapour take-up, industrial research in the footwear industry has been directed principally towards and concentrated on seeking to improve these two parameters. It is known that the magnitude of water vapour permeability of synthetic leathers is determined by the construction of the covering layer, that is to say the system comprising a covering film and the finishing film, because the carrier employed in this field, e.g. woven, looped or unwoven carrier, has itself good water vapour permeability characteristics.
Although a very large range of film-forming materials have been used in the synthetic leather industry, in the majority of cases a polyurethane or polyvinylchloride (P.V.C.) material has been used.
Researches conducted with the aim of increasing the water vapour permeability of synthetic upper materials in the footwear industry have led to significant results on the introduction of the so-called aqueous coagulation technology for polyurethane; thus several companies have commenced the manufacture of so-called synthetic uppers covered with porous polyurethane produced by this coagulation method.
The water vapour permeability of such materials can be regarded as better than the characteristics of previously manufactured synthetic uppers but as a consequence of their complicated production technology, and of other economic factors, they have not come into widespread use; indeed many companies have now stopped production.
The use of PVC as a film-forming material in the synthetic leather industry goes back a long time and is very widespread because its processing does not require a complicated technology. However, in regard to manufacturing of shoe uppers, only partial results have been achieved by the research aiming to improve water vapour permeability and thus the use of such uppers is restricted.
A satisfactory water vapour permeability for synthetic lining leathers provided with PVC covering layers can be achieved with relatively simple technological measures because the requirements made against such materials, e.g. resistance to repeated bending, physical and mechanical properties etc. are considerably less demanding than those made against uppers for the footwear industry.
The general requirement of ensuring good water vapour permeability in PVC films, may be achieved by numerous methods, but in the footwear industry and more particularly in the manufacture of synthetic leather uppers, relatively few processes are in actual use. One such process is the perforation (Footwear News 30, page 15, 1974) which is perhaps the oldest method still in use for providing suitable permeability properties. Its disadvantages are that the channels formed in the course of perforation may become blocked in use and that, due to the characteristics of the technology, the water resistance of the finished product is not satisfactory, a factor which is very significant consideration for upper materials.
Porous systems are also known which have been produced by sintering PVC. Although such systems have good water vapour permeability, other physical mechanical properties, especially their resistance against repeated bending is unsatisfactory, and consequently in the production technology for synthetic leather uppers the sintering process cannot be used because of these higher demands. (Kalosnikov V.G., Hrustoleva, I.M.: Kozhevenno-Obuvnaya Pvomyshlen nosty 20,57--59, 1978).
Another known process involves the use of a salt finely mixed with and dispersed in a PVC plastisol which salt can then be washed out from the film in a technological step following gel formation. In this way a relatively favourably water vapour permeable, continuous channel system is formed, see West German Pat. Nos: 833,805 and 1,444,154 as well as West German published application Nos: 1,111,375.
Furthermore, chemical and mechanical foaming processes are known which are the fundamental and most important methods of making PVC porous and which are also widely used in the manufacture of synthetic leather. In the case of chemical foaming, in dependence upon the choice of the system for softening the PVC and of the blowing agent, the gelled foam may have an open cell system or a closed-cell system or a mixed open and closed cell system. Insofar as the dissociation of the blowing agent and the generation of gas takes place above the gelling temperature, a closed cell system is formed which has practically no water vapour permeability. This technology is used in the manufacture of the traditional PVC-covered synthetic uppers wherein on top of the closed cell foam layer a solid PVC layer is formed which has suitable physical and mechanical properties and provides a suitable aesthetic appearance.
When the blowing agent employed dissociates below the gelling temperature then there results an essentially open-cell construction and the thus obtained product has numerous macropores on its surface which spoil the aesthetic appearance and do not enable the simultaneous achievement of good water vapour permeability and good water resistance; moreover, their resistance against repeated bending does not satisfy higher demands.
A further disadvantage of the chemical foaming technology is that it is very sensitive to local temperature non-uniformities in the gelation tunnel since it follows from the nature of the method that under the effect of temperature differences significant variations in film thickness may arise. A general survey of the chemical foaming processes may be found in the article by Schmidt, P. and Polte A., in Kunstsoffe, 57 (1967) pages 25-31 and Visnovsky: "British Plastics," 43, (1970), pages 90-93 and 109-112; SPE Journal, No. 28 (1972) pages 46 048.
Another important method of making PVC porous is the mechanical foaming process. The high pressure technology, such as the Elastomer, Dennis and Trovipor processes are not used in the synthetic leather industry because they are suitable principally for the production of thick layers, in contrast to the unpressurized mechanical methods which are suitable for forming open-pored structures (Sprague, Gr. Rubber and Plastics 34, page 415 (1955); Chemical Engineering 63, page 122, 124, 126 (1956); Modern Plastics 34, (1957) pages 117-118 and 212-213; Kunststoffe, 55 (1965), pages 717-723).
The essence of the mechanical foaming process which does not utilise pressure is that air, or possibly nitrogen, is dispersed in a plastisol containing a suitably selected surfactant (foam stabiliser), then the plastisol foam is spread by a suitable method and the thus obtained film is gelled. Bearing in mind that the foam formation takes place below the gelling temperature, the construction obtained in this way contains almost 100% open cells; local temperature differences in the gelation tunnel cause no significant thickness differences. These advantages of the above described technology have made mechanical foaming processes very attractive for the synthetic leather industry, see Acton J. Debal F.: Kunststoffe 62, (1972) pages 547-557; Schaum Kunststoffe pages 344-352 (1976); as well as West German Pat. Nos: 2,310,017 and 2,437,158.
The thus obtained film structure with open cells and good water vapour permeability has, however, relatively poor physical and mechanical properties; consequently, in hitherto known processes, a so-called wear layer formed mainly from polyurethane is applied to the top of the PVC film. Although on the one hand this measure ensured the achievement of suitable physical and mechanical properties, on the other hand, by its very nature it has significantly decreased the water vapour permeability; consequently, its area of applicability in the footwear industry has been reduced.